Plaice nurseries: effects on recruitment

The function of coastal nursery areas is discussed in relation to the variability in North Sea plaice recruitment. Since 60% of the recruitment of juveniles originates from the Wadden Sea, special attention is paid to this area. The concentration of juveniles in a restricted area seems to evoke only adverse effects: an increased risk of food limitation and hence reduced growth, and an increased vulnerability to predation. Despite these expectations, growth of most of the plaice in the Wadden Sea has always been optimal within the wide range of year-class strength observed and depends only on ambient water temperature. The same situation is indicated for some British bays, where the growth of 0-group plaice is far lower than in the Wadden Sea, because of lower temperatures. Mortality through predation seems to be relatively low in the Wadden Sea and restricted to only a short period, because of the absence of almost all potential predators. In the more open British bays higher mortalities are found, probably due to the presence of a number of predatory fish species. The low variability in the recruitment of plaice might be the combined result of optimal growth with the absence of between-year fluctuations in predator abundance in the Wadden Sea. As a result, observed mortality only depends on prey abundance and is therefore density-dependent both within one year and especially between years, reducing variations in recruitment. This suggestion is supported by the situation in more open British bays: here too, growth is maximal, but the abundance of predators shows larger fluctuations between years and, as a result, greater fluctuations in mortality are observed.